Systems which require multiple or cyclic chemical reactions to produce a desired product often require careful temperature control to produce optimal results. Such reactions include nucleic acid amplification reactions such as the polymerase chain reaction (PCR) and the ligase chain reaction (LCR). For this reason, apparatus have been developed which permit the accurate control of the temperature of reaction vessels in which such amplification reactions are performed.
For example, there are a number of thermal “cyclers” used for DNA amplification and sequencing in the prior art in which one or more temperature controlled elements or “blocks” hold the reaction mixture, and the temperature of a block is varied over time.
Another prior art system is represented by a temperature cycler in which multiple temperature controlled blocks are kept at different desired temperatures and a robotic arm is utilized to move reaction mixtures from block to block.
All of these systems include features which allow the user to program temperatures or temperature profiles over time for a block on the instrument so that various processes (e.g. denaturing, annealing and extension) can be efficiently accomplished once the optimum temperatures for these steps are determined. Importantly, however, the determination of the optimum temperature for each of the various steps in any reaction system, and in particular for any nucleic amplification or incubation reaction involving an annealing step, is not a simple task.
PCR is a technique involving multiple cycles that results in the geometric amplification of certain polynucleotide sequence each time a cycle is completed. The technique of PCR is well known to the person of average skill in the art of molecular biology. The technique of PCR is described in many books, including, PCR: A Practical Approach, M. J. McPherson, et al., IRL Press (1991), PCR Protocols: A Guide to Methods and Applications, by Innis, et al., Academic Press (1990), and PCR Technology: Principals and Applications for DNA Amplification, H. A. Erlich, Stockton Press (1989). PCR is also described in many U.S. patents, including U.S. Pat. Nos. 4,683,195; 4,683,202; 4,800,159; 4,965,188; 4,889,818; 5,075,216; 5,079,352; 5,104,792; 5,023,171; 5,091,310; and 5,066,584, which are hereby incorporated by reference.
The PCR technique typically involves the step of denaturing a polynucleotide, followed by the step of annealing at least a pair of primer oligonucleotides to the denatured polynucleotide, i.e., hybridizing the primer to the denatured polynucleotide template. After the annealing step, an enzyme with polymerase activity catalyzes synthesis of a new polynucleotide strand that incorporates the primer oligonucleotide and uses the original denatured polynucleotide as a synthesis template. This series of steps (denaturation, primer annealing, and primer extension) constitutes a PCR cycle. As cycles are repeated, the amount of newly synthesized polynucleotide increases geometrically because the newly synthesized polynucleotides from an earlier cycle can serve as templates for synthesis in subsequent cycles. Primer oligonucleotides are typically selected in pairs that can anneal to opposite strands of a given double-stranded polynucleotide sequence so that the region between the two annealing sites is amplified.
The temperature of the reaction mixture must be varied during a PCR cycle, and consequently varied many times during a multicycle PCR experiment. For example, denaturation of DNA typically takes place at around 90-95° C., annealing a primer to the denatured DNA is typically performed at around 40-60° C., and the step of extending the annealed primers with a polymerase is typically performed at around 70-75° C. Each of these steps has an optimal temperature for obtaining the desired result. Many experiments are required to determine the optimal temperature for each step.
For example, while the temperature at which DNA denatures is generally between 90-95° C., slight variations in the particular temperature necessary are observed depending on the length of the DNA and the percentage of each of the four deoxynucleotides present (guanine-cytosine pairs and adenine-thymine pairs). Insufficient heating during the denaturation step is a common reason for a PCR reaction to fail. However, overheating during the denaturation step can result in excessive denaturation of the polymerase.
Achieving the optimal temperature for the PCR annealing step is even more critical. An annealing temperature which is too low will result in non-specific DNA fragments being amplified. At too high of an annealing temperature, the primers will anneal less efficiently resulting in decreased yield of the desired product and possibly reduced purity. In the annealing step, the optimal temperature will depend on many factors including the length of the primer and the percentage of each of the four deoxynucleotides present (guanine-cytosine pairs and adenine-thymine pairs). For a typical 20-base oligonucleotide primer comprised of roughly 50% guanine-cytosine, a temperature of 55° C. is a good estimate for the lower end of the temperature range. However, as one increases the primer length in order to attain greater primer specificity, differing annealing temperatures may be required. Thus, the number of subtle influences on the optimal annealing temperature makes difficult the task of quickly identifying the optimum for a given system.
Achieving the optimal temperature for the extension reaction is also important for obtaining the desired PCR result. Temperature may affect both the rate and the accuracy of the extension reaction. If the rate of the polymerase reaction is too low, then the newly synthesized polynucleotide may not contain a site for primer annealing. Additionally, the denatured polynucleotide sequence for amplification may contain one or more regions of secondary structure that may form or disappear according to the temperature selected. Furthermore, several different enzymes with polymerase activity may be used for PCR. Each enzyme will have its own optimum temperature for activity, stability and accuracy.
Determination of the optimal denaturing, annealing, and extension temperatures for a particular PCR is complicated by the fact that the optimum will be different for each of the reactions. Thus, in order to determine the three optimal temperature ranges, multiple separate experiments must be run where two temperature variables are held constant while a third temperature variable is changed. As a result, determination of the optimal temperature for running a PCR system can be a time consuming task.
It is therefore an object of the present invention to provide an efficient means by which optimal reaction temperatures can be more efficiently identified for PCR and other reactions.